Genetic analysis and DNA repair studies will be used to examine the mechanisms of radiation mutagenesis in the lower eukaryote Saccharomyces cerevisiae. New alleles of genes involved in error-prone repair, such as the RAD6 locus, will be looked for and also searches conducted for new mutations that specifically block frame-shift mutagenesis or RAD6-dependent error-free repair. Genetic experiments will also be used to continue the study of "untargeted" mutagenesis and the mechanisms by which it occurs. DNA repair studies will examine the relationship between transdimer synthesis and mutagenesis, using a variety of mutant strains thought to be deficient in one or other of these processes, by means of DNA fiber autoradiography and alkaline sucrose gradient sedimentation.